Dog kidney and liver cytosol (105,000 X g supernatants) contain an acidic soluble protein of molecular weight 70,000 which reversibly binds thyroid hormones. K sub assoc of kidney cytosol protein (S protein) for T4 is 2.2 X 10 to the 9th power liter mole to the minus 1st power. Liver and kidney S protein is clearly distinct from dog plasma binding proteins for T4 and for triiodothyronine (T3). Kidney S.T4 complex emerges from an isoelectric focusing column at pH 4.2. S protein binds T3 less avidly than T4. Although S.T3 complex is focused at pH 5.1, available evidence suggests that a single S binds both T3 and T4, and that differences in isoionic points of the protein.hormone complexes reflect differences in the ligands (T4, T3). S protein does not bind iodotyrosines, steroids, or sulfobromophthalein (BSP). Purification of kidney S to a single stainable band on disc polyacrylamide gel electrophoresis has been effected with ammonium sulfate precipitation of cytosol, preparative isoelectric focusing and gel filtration. Current research objectives include studies of physical characteristics of purified dog kidney S, studies of antigenicity of S and investigation of possible interactions of S with particulate and nuclear fractions of kidney and liver cells.